The invention relates generally to hydrodynamic bearings including strain sensors and, more particularly, to hydrodynamic bearings containing fiber optic sensors for measuring static and dynamic bearing loads on the bearings during operation.
Hydrodynamic or fluid film bearings are widely employed as rotor supports in industrial machinery having high horsepower and high loads. A basic purpose of such bearings is to provide a relatively frictionless environment to support and guide a rotating shaft. Various designs for hydrodynamic bearings have evolved over time. Some bearings such as the lemon bore and pressure dam designs have a fixed geometry, without moving parts. In contrast, other bearings have a variable geometry, with the bearing pads flexing, pivoting or otherwise moving in response to changing load conditions in order to stabilize the bearings during use. Examples of variable geometry bearings include the ball-in-socket design, the rocker design, and the Orion(copyright) bearing. Another style variable geometry bearing is the Flexure Pivot(copyright) tilt-pad bearing, which is owned by the Assignee of the present application, and described in U.S. Pat. No. 5,513,917, the contents of which are incorporated herein by reference. Whether fixed or variable geometry, all hydrodynamic bearings share the common characteristic of having a cylinder which surrounds the rotating shaft and which is filled with some form of fluid lubricant, such as oil. The fluid is the medium that supports the shaft to prevent metal to metal contact. As the shaft rotates, an oil wedge is created that supports the shaft and relocates it within the bearing clearances.
Hydrodynamic bearings have many differing designs to compensate for differences in load requirements, machine speeds, cost, and/or dynamic properties. Regardless of the design of the hydrodynamic bearing, as the shaft rotates imbalance and eccentricity can cause the shaft to exert a dynamic load on the bearing, particularly on the bearing pads. This can lead to fretting wear on the pad support surfaces and fatigue of the babbit metal of the bearing, and eventually lead to bearing destruction which, in turn, can cause catastrophic failure. Prior art attempts to measure the loads on hydrodynamic bearings during use have not been successful due to various factors. In particular, failures have resulted from the environmental conditions under which the bearings operate (namely high temperatures, high pressures and moisture), and from the design of the measuring devices themselves which, when installed, affected bearing performance. One example at a past attempt to measure the dynamic loads or forces on hydrodynamic bearings during operation is disclosed in U.S. Pat. No. 4,027,539 to Halloran. The Halloran patent discloses the use of two force probes mounted on the bearing to sense the force components in two perpendicular directions. A piezo-electric load cell in each force probe measures the frequency and a proportional amount of the amplitude of the force component in its direction during operation, and converts the sensed frequency and amplitude into an electrical signal. The sensed amplitude in each direction is calibrated to actual amplitude and the calibrated signals are displayed on an oscilloscope. In use, the mounting of the Halloran device can, itself, adversely affect bearing performance. In addition, the piezo-electric load cell is vulnerable to failure in high temperature environments. Thus, dynamic load sensors (an example of which is the Halloran device) are not currently utilized in the industry.
Bearings can also become subject to instability which manifests itself as oil whirl or oil whip. Left uncorrected, this phenomenon is catastrophic and can destroy the bearings, seals, and the rotor very quickly. Oil whirl normally occurs in lightly loaded bearings, where the oil whirling forces, which are usually manifested at about 42% to 48% of rotor speed, begin to dominate and actually carry the rotor in the direction of rotation. If the rotor speed increases to a point where the oil whirl frequency coincides with the rotor""s first natural frequency, oil whip occurs. Oil whip is a dangerous condition where rotor damping is unable to limit the rotor""s motion so that displacement amplitudes continue to grow until halted by contact with stationary parts, such as internal seals. Once this type of internal contact exists, the rotor begins to precess, in a reverse direction from rotor rotation direction, using the entire bearing clearance. This condition leads to high friction levels which will overheat the babbit metal and leads to rapid destruction of the bearing, rotor journal, and the machine seals.
In order to prevent such occurrences, proximity probes have also been developed to monitor the relative motion between the shaft and the bearing. Proximity probes have been utilized to measure the relative vibration of the shaft as well as the relative position of the shaft with respect to the bearing clearances (i.e. displacement). Although these probes have been useful, they suffer from significant drawbacks. Such probes are costly to install in machines and may not be able to be installed in all machines. In addition, they are subject to run out problems because the effectiveness of the probes (which are electromagnetic) relies on the continuity of the surface of the shaft. Even small imperfections in the surface of the shaft can lead to false read outs, especially at high speeds. Also, the proximity probes are often permanently damaged by their operating environment (high temperatures, forces, etc.) and are costly to replace. These sensors can also be unreliable due to their operating environment, and further lack the sensitivity to detect damaging levels of vibration at speeds above 40,000 to 50,000 rpm. The disadvantages with such probes increases when trying to use the probes with small rotors, such as those found in turbo chargers and micro turbines, where it becomes increasingly difficult to accurately measure displacement with the probes. Such probes also do not measure the dynamic loads which can cause bearings to wear, as discussed above.
Accordingly, there is needed in the art a sensor which can measure dynamic forces or loading of a bearing during operation in a reliable manner, and which may also detect unwanted displacement of the shaft and bearing prior to catastrophic failure.
A hydrodynamic bearing including a fiber optic sensor for measuring static and dynamic bearing forces or loads during operation which is cost effective to use, easily installed and reliable during operation is disclosed herein. In one embodiment, the fiber optic sensor is a fiber Fabrey Perot interferometor sensor having internal mirrors spaced from each other within a fiber, as described below.
The sensor is disposed within the bearing through the most direct load path, which is through the pad support for a tilting pad style bearing. The sensor may be disposed in one, more than one, or all of the pad supports, depending upon the particular application. In addition to being disposed in the pad support of a tilting pad bearing, the sensor should also be placed 1) inside the pad support or on the pad support structure, and 2) oriented perpendicularly with the shaft centerline, xe2x80x9ccxe2x80x9d. After the sensor location is chosen and the sensor is properly positioned, a calibration procedure is utilized to determine the relationship between the radial load and measured strain for the specific bearing. Once the calibration factor has been determined, the sensor may be utilized in the bearing to measure load during operation, as described herein below. In one embodiment disclosed herein, the sensor is utilized in a Flexure Pivot(copyright) bearing, while in a second illustrative embodiment, the sensor is disposed in a ball-in-socket bearing.